This disclosure relates generally to equipment utilized and operations performed in conjunction with a subterranean well and, in an example described below, more particularly provides a pressure cycle operated perforating firing head.
It is very important that a firing head used, for example, to initiate explosives in a perforating gun is reliable and safe in operation. Many firing head designs have been proposed in the past, some of which operate in response to pressure applied to the firing head from a remote location. Unfortunately, these past designs have suffered from one or more significant drawbacks.
For example, most pressure operated firing heads rely on shear pins to select a pressure which, when applied to the firing head, shears the pins and initiates a detonation sequence, with or without a built-in delay. One disadvantage of these firing heads is that a large number of shear pins must be installed in order to select a correspondingly high actuation pressure, but each shear pin has an inherent shear value inaccuracy (e.g., due to variations in size, material composition, heat treatment, etc.), and these inaccuracies accumulate, with the result that high actuation pressures also have high inaccuracies.
Another disadvantage of these firing heads is that they typically include a chamber which is pressurized such that, either at the surface or downhole, a very large pressure differential exists between the chamber and the surrounding environment. For example, an atmospheric (or other relatively low pressure) chamber must be surrounded with a thick wall in order to withstand downhole pressures. On the other hand, a chamber which is pressurized (for example, with nitrogen) to a thousand or more psi (≧7000 kPa) at the surface not only requires a substantial wall surrounding the chamber, but also presents hazards to the personnel who must pressurize the chamber at the surface, handle and install the firing head after pressurization, etc.
Therefore, it may be seen that improvements are needed in the art of pressure operated firing heads. These improvements may also be useful in other operations, as well, such as in generating electricity downhole, etc.